Capsule-enclosed electro-acoustic transducer and transistor amplifier



Aug. 17, 1965 B. WEINGARTNER 3,201,516 CAPSULE-ENCLOSED EL RO-ACOUSTIC TRANSDUCER IER AND TRANS OR AMPLIF Filed May 14, 1962 INVENTOR BER 1m RD wsm e-AR fl/E1? BY Md. JA

ATTORNEYS United States Patent 3,2M,5i6 CAPSULE ENCLGSED ELEQTRG-ACQUS- TlC TRANSDUQER AND TRANSETUR AMPLIFIER Bernhard Weiugartner, Vienna, Austria, assignor to Airtistische U. Kino-Gerate Geseilschaft m.h.H., Vienna, Austria, 21 firnl Filed May 14, 1962, Ser. No. 194,265 Claims priority, application Austria, Feb. 3, 1962, A 907/62 13 Claims. (Cl. 179-1) This invention relates to an electro-acoustic transducer incorporated in a capsule and comprising a transistor amplifier, particularly for telephone handsets, which transducer, which is in most cases of electro-dynamic type, is of midget size and disposed approximately at the center of an annular disc which is disposed in the capsule and serves as a mounting panel and, if desired, as a circuit panel, for a transistor amplifier.

This arrangement, which is known under the name transistor microphone has the same external dimensions as the previously used carbon microphones so that a direct replacement is possible. Vhereas the transistor microphones have special advantages, such as freedom from distortion, long life etc., they have an important disadvantage compared to carbon microphones in that they they are highly sensitive to interfering noise. The carbon microphone is known to have a response threshold such that acoustic signals (sound waves) on a level below a certain threshold cannot be received by the microphone. As a result, interfering noise, the sound pressure level of which is generally much lower than the signal level, is not transmitted because the vibrating mechanism causes a virtually automatic suppression of noise.

This is not the case with the usual dynamic or magnetic transducers which will transmit interfering and accompanying noise even on a very low level, particularly when they have a uniform sensitivity in all directions. This has an adverse effect on the intelligibility of the information to be transmitted.

It is an object of the invention to eliminate the disadvantages of these known transistor microphones while enabling the universal applicability thereof in the usual telephone handsets. According to a first feature of the invention, this is achieved in that the acoustic transducer, more particularly electro-dynamic transducer, which is incorporated in the capsule, is designed as a noise-compensating pressure gradient transmitter, the sensitivity of which decreases toward the low frequencies in a plane sound field (noise field) whereas it varies substantially linearly with frequency in a spherical sound field (signal According to another feature of the invention, the electro-acoustic transducer incorporated in the capsule is characterized in that the noise-compensating transmitter is a higher-order pressure gradient transmitter of higher order, preferably of second order, and has associated with it a tore-shaped directional pattern, which is obtained by the rotation of an approximately figure eight-shaped, plane figure about its center line at right angles to the longitudinal axis.

According to the invention, this is achieved by the provision of coupling volumes disposed before and behind the diaphragm and communicating with the open air by one or more ducts of any desired form and of such lengths that the paths of sound from any possible sound entrance point to both coupling Volumes are at least approximately equal, whereas the joint impedance of the duct or ducts extending from the front side or" the diaphragm is at least approximately equal to the inn pedance of the duct or ducts extending from the rear side of the diaphragm.

movements are mainly controlled by friction.

It is apparent from the foregoing that the attenuation of the noise from remote sources is primarily elfected according to the invention, by the acoustic compensation of the remote field, and that this compensation is considerably assisted by additional features as a result of which the directional pattern is such that the axis of main sensitivity thereof extends toward the mouth of the speaker.

The invention will be explained more fully with reference to the accompanying drawings, in which FIG. 1 shows a usual directional pattern and PEG. 2 the directional pattern of a transducer according to the invention.

FIGS. 3 and 5 are sectional views taken through two illustrative embodiments and FIGS. 4 and 6 are corresponding top plan views.

A transducer in which the invention may be embodied and which has the required properties, is the known transmitter which is responsive to the particle velocity and the resonant frequency of which lies in the middle of the frequency range to be transmitted, whereas the diaphragm In that case the force acting on the diaphragm increases in proportion with frequency in a plane sound field at least as long as the dimensions of the diaphragm are less than the wavelength of the highest frequency to be transmitted. In a plane sound field, as is constituted in most cases by the noise field owing to its relatively large distance from the sound transmitter, the sensitivity decreases by 6 decibels per octave in dependence on frequency below a limiting frequency, which is determined by the sound detour, i.e., by the sound path from the center of the front side to the center of the rear side of the diaphragm.

When such a microphone is voice-actuated from a short distance, i.e., in a spherical sound field, in which the sound energy decreases as the square of the distance from the sound source, the diaphragm will be mainly actuated by the direct sound. The resulting increase in pressure difference toward the low frequencies results in a linearization of the frequency response of the capsule. This effect is much improved by the circuit panel for the transistor amplifier, which panel serves as a battle.

Another increase in effect regarding the suppression of noise will be obtained, according to the invention, if the transducer described hereinbefore is given a special directional pattern. Thorough investigations have shown that it is not sufficient to give the transducer one of the usual directional patterns and that the directional pattern must meet specific requirements to reduce the sensitivity to interfering noise and to enable the incorporation of the capsule in any desired position.

The three-dimensional directional pattern of the previously known systems has the shape or" a figure eight, which becomes narrower as the ordinal number increases and which rotates about its longitudinal axis. In a system of first order, this results in a directional pattern which can be represented by two contiguous spheres (FIG. 1). This pattern and that of systems of higher order has the disadvantage that in the planes of section for which zconstant the sensitivity varies according to a figure eight-shaped pattern and that the sensitivity is theoretically zero in the plane of section z:0. A capsule provided with such a transducer cannot be incorporated in the handset in any desired position but the position must be such that the direction of the axis of highest sensitivity (z axis) coincides with the longitudinal axis of the handset.

The system proposed by the invention, which is of second order and comprises one diaphragm and has a toreshaped directional pattern, is free from this disadvantage.

FIG. 2 shows the directional pattern in a three-dimen-l sional showing, a portion being cut out for a better representation of the transverse sectional shape. The shape may be considered obtained by the rotation of a figure eight, which is slenderer because the system is of higher (second) order, about an axis which extends at right angles to the longitudinal axis and through the center, the z axis (FIG. 2) coinciding with the axis of the trans ducer.

A transducer having such a pattern as contrasted to the usual form shown in FIG. 1 has the lowest sensitivity in the direction of the z axis, whereby the possibility of a reception of noise from the space is further restricted. The highest sensitivity is in a plane extending at right angles to the longitudinal axis of the handset.

The structure of such a transducer is shown in FTGS. 3 to 6. As is apparent from, FIG. 3 the vibratory system actuated by the sound comprises the circular diaphragm 1 and the moving coil 2 moving in the air gap of a magnet system. Coupling volumes 3 of equal size are provided before and behind the diaphragm. Each of these cavities has one or more sound ducts connected to it. A duct 4 having an opening exactly in the axis of the transducer system leads into the open from the front side of the diaphragm. The length of this duct is equal to the length of the sound path from the rear side of the diaphragm into the open and the acoustic impedance of this duct equals the acoustic impedance of all sound ducts leading to the rear side of the diaphragm. At least four sound ducts 6 extend into the open from. the rear side of the diaphragm and have openings 7 disposed in the same plane as the opening 5 of duct 4. The ducts 6 form corners of a square, through the center of which the axis of the transducer extends.

The carrier panel 8 is suitably made from coppercoated insulating plastic for receiving the printed circuit. In addition to the transducer, the panel carries the electrical circuit elements 9, such as transistors, resistors, capacitors etc. The housing 10 provided with the cover 11 has the same standardized dimensions as a carbon microphone.

The acoustic function of the arrangement will be described with reference to FIG. 4. Sound pressures p to p occur at the various openings. Sound which is incident in a direction normal to the plane of the openings gives rise to sound pressures which are equal in magnitude and in phase. If the sound ducts extending from the chambers on the front and rear sides of the diaphragm are of equal size and the sound duct opening at 5 has the same acoustic impedance as the sound ducts ending at the openings 7, the actuating forces will cancel each other at the diaphragm. so that the same remains stationary. When the microphone is voice-actuated from directions 0 or 180, the actuating force at the diaphragm will be proportional to the difference between the pressure differences. Pressures p and p;; are effective at the rear side and pressure 11 is effective at the front side of the diaphragm. Hence, the resulting transducer is a pressure gradient microphone of second order. In the case of sound incidence from directions 90 and 270, an analogous result will be obtained, namely a pressure gradient microphone of second order will be analogously obtained if the absolute value of the sound pressures and the distances of the openings 7 from the center are of equal size. It will be readily apparent that a cos pattern will also be obtained for voice actuation from all intermediate directions. As a result, the directional pattern will be torelike and axially symmetrical with respect to the z axis and its meridian section will be a cos s figure. The mechanical impedance of the vibratory system should be selected so that the frequency response of the transmission level in the plane sound field decreases by 6 decibels per octave toward the low frequencies. This will result in an approximately linear frequency response in a spherical sound field. For a dynamic pressure gradient microphone of second order, the mechanical impedance should be mainly a mass, so that the natural frequency of the vibratory system should be at the lower end of the frequency range to be transmitted (mass control).

Instead of the dynamic transducer system, any desired other system, e.g., a magnetic, piezoelectric or capacitive system may be coupled to the diaphragm.

FIG. 5 shows a modified embodiment of FIG. 3. Analogous references are used. More than four openings 7 may lead to the rear side of the diaphragm. Separate openings may be replaced by an annular passage, which may be intersected by tubular sound ducts. It is essential that the sound paths from any possible sound entrance point to the coupling volumes before and behind the diaphragm are of equal length and that both sound paths have the same acoustic impedance. To obtain directional patterns deviating from the pure cos 3: shape the lengths of the sound paths to the front and rear side of the diaphragm may differ slightly.

What is claimed is:

1. An electro-acoustic transducer assembly comprising, in combination, an encasing capsule; an annular disc extending completely across said capsule; a transistor amplifier having its components mounted on said disc; a noise compensating pressure gradient transducer disposed substantially within the center opening of said disc; means providing a pair of substantially equal coupling volumes each communicating with respective different faces of said transducer; and duct means communicating with each of said coupling volumes and having air openings located in a common plane substantially parallel to said transducer; said transducer having a sensitivity decreasing with sound frequency for sound waves having a plane sound field at the location of said transducer, and having a sensitivity varying substantially linearly with sound frequency for sound waves having a spherical field.

2. A transducer assembly as set forth in claim l, which is incorporated in a telephone handset.

3. A transducer assembly as set forth in claim 1, in which said transducer is a midget size electro-dynamic transmitter.

4-. A transducer assembly as set forth in claim 1, in which said disc forms a circuit panel for said transistor amplifier.

5. A transducer assembly as set forth in claim 1, in which said transducer has a dynamic pressure gradient of an order higher than one, and has a tore-shaped directional pattern which is obtained when an approximately figure eight-shaped, plane figure, having a longitudinal axis and a center line extending at right angles to the longitudinal axis is rotated about said center line.

6. A transducer assembly as set forth in claim 5, in which said dynamic pressure-gradient is of second order.

'7. An electro-acoustic transducer assembly, as claimed in claim 1, in which said transducer includes a diaphragm having a front face and a rear face; said means providing a pair of substantially equal coupling volumes comprising a first coupling air chamber in communication with said front face and a second coupling air chamber in communication with said rear face; said duct means including first duct means connecting said first chamber to the open air and second duct means connecting said second chamber to the open air; said duct means having lengths such that the paths of sound from any possible sound entry point to both of said chambers are substantially equal, and said first duct means and said second duct means having approximately equal acoustic impedance values; the air openings of the duct means communicating with the first coupling air chamber being located in an inner zone substantially centrally of said common plane; and an outer zone of said common plane surroundig said central zone, the air openings of said second duct means being located in said outer zone.

8. A transducer assembly as set forth in claim 7, in which said diaphragm is circular, said chambers are of equal size and said first and second duct means are of which said second duct means comprises four ducts of 10 2,228,886

equal length.

10. A transducer assembly as set forth in claim 9, in which said openings of said four ducts form corners of a square, which is centered on the axis of said transmi-tter.

11. A transducer assembly as set forth in claim 7, in which said first duct means comprise a body disposed in front of said diaphragm and defining a single, winding duct connecting said first chamber to the open air.

12. A transducer assembly as set forth in claim 7, in which said first duct means comprises a single, spiral duct connecting said first chamber to the open air.

13. A transducer assembly as set forth in claim 7, in which said second duct means comprises an annular duct connecting said second chamber to the open air.

References Cited by the Examiner UNITED STATES PATENTS 1/41 Olson l791 2,299,620 10/42 Giannini 179-138 2,463,762 3/49 Giannini 179-179 2,870,255 1/59 Jenkins et a1. 179121 3,048,659 8/62 Craw et a1. 179-81 FOREIGN PATENTS 1,014,595 8/57 Germany. 190,988 7/57 Austria.

ROBERT H. ROSE, Primary Examiner. 

1. AN ELECTRO-ACOUSTIC TRANSDUCER ASSEMBLY COMPRISING, IN COMBINATION, AN ENCASING CAPSULE; AN ANNULAR DISC EXTENDING COMPLETELY ACROSS SAID CAPSULE; A TRANSISTOR AMPLIFIER HAVING ITS COMPONENTS MOUNTED ON SAID DISC; A NOISE COMPENSATING PRESSURE GRADIENT TRANSDUCER DISPOSED SUBSTANTIALLY WITHIN THE CENTER OPENING OF SAID DISC; MEANS PROVIDING A PAIR OF SUBSTANTIALLY EQUAL COUPLING VOLUMES EACH COMMUNICATING WITH RESPECTIVE DIFFERENT FACES OF SAID TRANSDUCER; AND DUCT MEANS COMMUNICATING WITH EACH OF SAID COUPLING VOLUMES AND HAVING AIR OPENINGS LOCATED IN A COMMON PLANE SUBSTANTIALLY PARALLEL TO SAID TRANSDUCER; SAID TRANSDUCER HAVING A SENSITIVITY DECREASING WITH SOUND FREQUENCY FOR SOUND WAVES HAVING A PLANE SOUND FIELD AT THE LOCATION OF SAID TRANSDUCER, AND HAVING A SENSITIVITY VARYING SUBSTANTIALLY WITH SOUND FREQUENCY FOR SOUND WAVES HAVING A SPHERICAL FIELD. 